Most portable electronic devices such as laptop and notebook computers, mobile phones and game devices have viewing screens unlike the cathode-ray-tube (CRT) monitors of conventional desktop computers. Users generally expect the viewing screens to provide performance equal to that of CRT monitors. To meet this demand, computer manufacturers have sought to build flat panel displays (FPDs) offering superior resolution, color and contrast, while at the same time requiring minimal power consumption. LCDs are one type of FPD which satisfy these expectations. However, the liquid crystals of an LCD are not self-luminescent. Rather, the LCD generally needs a surface emitting device such as backlight module which offers sufficient luminance (brightness) in a wide variety of ambient light environments.
As shown in FIG. 7, a typical backlight module 10 includes a reflector 11, a planar light guide plate 12, a second diffuser 13, a prism sheet 14, a first diffuser 15, two light sources 16 disposed at two opposite sides of the planar light guide plate 12. The reflector 11, the planar light guide plate 12, the second diffuser 13, the prism sheet 14 and the first diffuser 15 are stacked together in that order from bottom to top. The light guide plate 12 includes a light output surface 122, a bottom surface 123 opposite to the light output surface 122, and two light incident surfaces 121 interconnecting the light output surface 122 and the bottom surface 123. The reflector 11 is disposed adjacent to the bottom surface 123. The second diffuser 13, the prism sheet 14 and the first diffuser 15 are disposed upon the light output surface 122 in that order. The two light sources 16 are disposed opposite to the two light incident surfaces 123, respectively.
In operation, light beams emitted from the light sources 16 enter the light guide plate 12. Some of the light beams are reflected and then exit through the output surface 122, and other light beams directly exit through the output surface 122. All of the light beams that exit through the output surface 122 then transmit through the second diffuser 13, the prism sheet 14 and the first diffuser 15, and finally illuminate a liquid crystal panel (not shown). The first and the second diffuser 15, 13 are respectively used to scatter light beams transmitting therethrough, and the prism sheet 14 is used to control light beams transmitting therethrough to emit out along a predetermined direction generally perpendicular to the liquid crystal panel, and converge the light beams to a central region of the backlight module 10. The first and the second diffusers 15, 13 each have a haze ratio. Generally, the haze ratio of the first diffuser 15 is larger than that of the second diffuser 13 for assuring the utilization ratio of the light beams.
However, when the two light sources 16 are disposed at two opposite sides of the light guide plate 12, a brighter region may be created at the central region of the backlight module 10. In addition, the convergence function of the prism sheet 14 further enhances the brightness of the central region of the backlight module 10. Thus, the backlight module 10 has a brighter central region and a comparatively darker peripheral region. Therefore, the backlight module 10 can not produce a uniform surface light beams output, which lowers the display quantity, reduce the viewing angle.
What is needed, therefore, is a backlight module and a liquid crystal display which can provide uniform surface light beams output.